Induction sealing device for heat sealing packaging material

ABSTRACT

There is described an induction sealing device for heat sealing packaging material for producing sealed packages of pourable food product and fed in a first direction. The sealing device has induction means interacting with the packaging material by means of a pair of first active surfaces and a pair of second active surfaces interposed between the first active surfaces in the first direction; and the second active surfaces have, in the first direction, a width greater than the width of the first active surfaces at least along most of the length of the first and second active surfaces in a second direction perpendicular to the first direction.

This is a U.S. National Stage of International application No.PCT/EP02/06659, filed on Jun. 17, 2002.

TECHNICAL FIELD

The present invention relates to an induction sealing device for heatsealing packaging material for producing sealed packages of pourablefood products.

The present invention also relates to a packaging unit for continuouslyproducing sealed packages of a pourable food product and featuring saidsealing device.

BACKGROUND ART

Many pourable food products, such as fruit juice, UHT milk, wine, tomatosauce, etc., are sold in packages made of sterilized packaging material.

A typical example of such a package is the parallelepiped-shaped packagefor liquid or pourable food products known as Tetra Brik Aseptic(registered trademark), which is formed by folding and sealing laminatedstrip packaging material having a multilayer structure comprising alayer of fibrous material, e.g. paper, covered on both sides with layersof heat-seal plastic material, e.g. polyethylene.

In the case of aseptic packages of long-storage products such as UHTmilk, the packaging material has a layer of barrier material, e.g. analuminium sheet, which is superimposed on a layer of heat-seal plasticmaterial and is in turn covered with another layer of heat-seal plasticmaterial eventually defining the inner face of the package contactingthe food product.

As is known, such packages are made on fully automatic packaging units,on which a continuous tube is formed from the web-fed packagingmaterial; the web of packaging material is sterilized on the packagingunit itself, e.g. by applying a chemical sterilizing agent, such as ahydrogen peroxide solution, which, after sterilization, is removed, e.g.vaporized by heating, from the surfaces of the packaging material; andthe web of packaging material so sterilized is maintained in a closedsterile environment, and is folded and sealed longitudinally to form avertical tube.

The tube is fed continuously in a first vertical direction, is filledwith the sterilized or sterile-processed food product, and is gripped atequally spaced cross sections by two pairs of jaws. More specifically,the pairs of jaws act cyclically and successively on the tube to heatseal the packaging material of the tube and form a continuous strip ofpillow packs connected to one another by respective transverse sealingbands, i.e. extending in a second direction perpendicular to said firstdirection.

The pillow packs are separated by cutting the relative transversesealing bands, and are then conveyed to a final folding station wherethey are folded mechanically into the finished parallelepiped shape.

The portion of the tube gripped between each pair of jaws is heat sealedby heating means carried on one of the jaws and for locally melting thetwo layers of heat-seal plastic material gripped firmly between thejaws.

More specifically, when the layer of barrier material is defined by asheet of electrically conductive material, e.g. aluminium, the packagingmaterial is normally sealed using a so-called induction heat-sealprocess, in which, when the tube is gripped by the jaws, eddy current isinduced in the aluminium sheet to heat the aluminium sheet locally andso locally melt the heat-seal plastic material.

More specifically, in induction heat sealing, the heating meanssubstantially comprise an inductor, which is carried by one of the twojaws, known as the sealing jaw, is supplied by a high-frequency currentgenerator, and is substantially defined by one or more induction barsmade of electrically conductive material, extending parallel to thesecond direction, and interacting with the tube material to induce eddycurrent in and heat the material to the required sealing temperature.The other jaw, known as a counterjaw, has pressure pads of elastomericmaterial, which cooperate with the induction bars to heat seal the tubealong a respective transverse sealing band.

At the end of the sealing operation, a cutting member, carried by one ofthe two jaws, normally the counterjaw, and interacting with the tube ofpackaging material, is activated to cut the tube along the centerline ofthe transverse sealing band and so cut a pillow pack off the bottom endof the tube of packaging material. The bottom end is therefore sealedtransversely, and the jaws, on reaching the bottom dead center position,are opened to avoid interfering with the top portion of the tube. At thesame time, the other pair of jaws, operated in the same way, moves downfrom the top dead center position and repeats the gripping/forming,sealing and cutting operations described above.

From analysis of the packaging material during the heat-seal operation,the eddy current induced in the cross section of the tube of packagingmaterial gripped between a respective pair of jaws has been found totravel along an endless path, which is linear on the two longitudinalsides of the inductor-tube interaction portion, i.e. along the sidesparallel to the second direction, and is roughly semicircular close tothe edges of the cross section. In other words, the current travelslinearly in opposite directions along the two longitudinal sides of theinductor-tube interaction portion, and, close to the edges of the crosssection gripped between the jaws, deflects towards the center of thecross section (“bending-off effect”), so that the transverse sealingband is narrower at the ends than at the center portion, i.e. theportion intersecting the longitudinal seal initially formed to producethe tube of packaging material. Moreover, when packaging pourable foodproducts containing small solid particles (such as seeds in tomatoproducts) which may get trapped between the unsealed portions of the twocontacting sheets of the packaging material, as wide a transversesealing band as possible is desirable to reduce the likelihood ofchanneling through the sealed portion.

To eliminate the above drawbacks, Patent Application EP0992431, filed bythe present Applicant, proposes that each cross section of the tube ofpackaging material be cut before being sealed.

As described in the above European patent, inverting the cutting andsealing operations produces a variation in the path of the eddy currentsinduced in the cross sections of the tube of packaging material. Thatis, the parting line produced by the cutting member in the tube ofpackaging material interrupts the electric continuity of the aluminiumsheet, so that the eddy currents induced by the induction bars in thepackaging material are confined to opposite sides of the parting line.In other words, the eddy current induced in the packaging material bythe induction bars on one side of the parting line tends towards theinduction bars on the opposite side of the parting line, but, owing tothe interruption in the packaging material, is forced to complete aclosed path on the same side of the parting line.

The sealing area on both sides of the parting line is therefore more orless constant, by drastically reducing the bending-off effect of theeddy current close to the edges of the cross section of the tube grippedbetween the jaws.

Number 50 in FIG. 6 indicates as a whole one example of a known sealingjaw which may be used in particular on packaging units in which thecross sections of the tube of packaging material are cut before beinginduction sealed.

Jaw 50 has a plane M of symmetry perpendicular to the travelingdirection of the tube of packaging material, and comprises two inductionelements 51, 52 housed inside respective face seats on jaw 50 andinteracting with the packaging material via respective pairs of activesurfaces 53, 54.

More specifically, induction element 51 is U-shaped, has a substantiallyannular cross section, and externally defines the two active surfaces53, which are located symmetrically on opposite sides of plane M.Induction element 52 is defined by a straight bar having a U-shapedcross section, housed along the middle of jaw 50, and defining the twoactive surfaces 54, which are located on opposite sides of plane M andbetween active surfaces 53.

Active surfaces 53, 54 all have continuous or segmented longitudinalprojections projecting towards the packaging material.

When using sealing jaw 50, the eddy current induced in the packagingmaterial has been found to travel along endless, substantiallysymmetrical paths on opposite sides of the parting line produced by thecutting member. More specifically, on each side of the parting line, theeddy current travels linearly along each active surface, and deflectsminimally close to the edges of the packaging material; which deflectiononly affects a small part of the sealing portion and is thereforenegligible.

While providing for wider transverse sealing bands than those obtainablewithout inverting the cutting and sealing operations, sealing jaw 50described above has several minor drawbacks preventing its manyadvantages from being used to the full.

In particular, FIG. 7 shows a graph of the power induced (continuousline) and the temperature (dash line) in the aluminium layer of thecross section of the tube of packaging material interacting with jaw 50as a function of the distance from plane M or, equally, from the partingline of the cross section.

The FIG. 7 graph refers to only one of the two halves into which thecross section of the tube of packaging material interacting with jaw 50is ideally divided by plane M, it being understood that the inducedpower and temperature curves in the other half of the cross section areperfectly symmetrical with those shown with respect to plane M.

As shown in the FIG. 7 graph, given the geometry of sealing jaw 50, andin particular of active surfaces 53, 54 of induction elements 51, 52,the power induced in the aluminium layer reaches a peak P1* at plane Mand a peak P2* at each active surface 53, and assumes a minimum valuebetween each active surface 54 and the adjacent active surface 53. Morespecifically, peak P2* is much lower than peak P1*.

Temperature distribution along the aluminium layer of the packagingmaterial is a direct result of the induced power distribution producedby the dimensional ratios of active surfaces 53, 54 of inductionelements 51, 52.

More specifically, moving in the traveling direction of the packagingmaterial away from plane M or, equally, from the parting line producedin the cross section by the cutting member, the temperature in thealuminium layer of the cross section of the tube of packaging materialinteracting with jaw 50 falls sharply (by about 40%).

The above temperature distribution results in overheating of the layersof heat-seal plastic material at the mid longitudinal portion of thetransverse sealing band, so that the molten material gripped between thejaws tends to flow outwards of the sealing band, thus impairing thequality of the seal.

To make up for the low power induced in the eddy current deflectionregions, which results in reduced heating of the packaging material, asdeducible from FIGS. 6 and 7 combined, sealing jaw 50 must be equipped,at the current deflecting regions, with inserts made ofmagnetic-flux-concentrating material, such as ferrite-containingcomposite material; and similar inserts must be provided on jaw 50 atthe intersection between the transverse sealing band and thelongitudinal seal on the tube of packaging material. In fact, at saidintersection, where the packaging material for sealing is thicker owingto the presence of three superimposed portions of material, the sealingquality obtainable, without the inserts, in the lesser heated portion ofthe packaging material is far from satisfactory.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an induction sealingdevice designed to eliminate the aforementioned drawback.

According to the present invention, there is provided an inductionsealing device for heat sealing a packaging material for producingsealed packages of pourable food products.

It is a further object of the present invention to provide a packagingunit for producing sealed packages of a pourable food product from atube of packaging material, and which provides for improved transversesealing of the packaging material as compared with known packagingunits.

According to the present invention, there is provided a packaging unitfor producing sealed packages of a pourable food product from a tube ofpackaging material.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred, non-limiting embodiment of the present invention will bedescribed by way of example with reference to the accompanying drawings,in which:

FIG. 1 shows a side view, with parts removed for clarity, of a packagingunit for producing aseptic sealed packages of pourable food productsfrom a tube of packaging material fed along a vertical forming path;

FIG. 2 shows a larger-scale side view of a cutting member of the FIG. 1packaging unit;

FIG. 3 shows a cross section of an induction sealing jaw in accordancewith the present invention and forming part of the FIG. 1 packagingunit;

FIG. 4 shows a graph of the power induced and the temperature in thealuminium layer of the packaging material interacting with the FIG. 3sealing jaw as a function of the distance from a mid-plane of the jawperpendicular to the forming path;

FIG. 5 shows an equivalent electric diagram of the FIG. 3 sealing jaw;

FIG. 6 shows a cross section of a known induction sealing jaw;

FIG. 7 shows a graph of the power induced and the temperature in thealuminium layer of the packaging material interacting with the FIG. 6sealing jaw as a function of the distance from a mid-plane of the jawperpendicular to the forming path.

BEST MODE FOR CARRYING OUT THE INVENTION

Number 1 in FIG. 1 indicates as a whole a packaging unit for producingaseptic sealed packages 2 of a pourable food product, such aspasteurized or UHT milk, fruit juice, wine, etc., from a tube 3 ofpackaging material.

The packaging material has a multilayer structure (not shown) andcomprises a layer of fibrous material, normally paper, covered on bothsides with respective layers of heat-seal plastic material, e.g.polyethylene; and the side of the packaging material eventuallycontacting the food product in package 2 also has a layer ofelectrically conductive barrier material, e.g. aluminium, in turncovered with one or more layers of heat-seal plastic material.

Tube 3 is formed in known manner upstream from unit 1 by longitudinallyfolding and sealing a web of heat-seal sheet material, is filled withthe sterilized or sterile-processed food product for packaging, and isfed by known devices (not shown) along a vertical path defined by adirection A.

Unit 1 comprises a pair of forming assemblies 4 movable vertically alongrespective guides (not shown) and which interact cyclically andsuccessively with tube 3 to grip, induction heat seal, and cut tube 3 atequally spaced cross sections.

Each forming assembly 4 substantially comprises a slide (not shown)running along the respective guide; and two jaws 5, 6 (shown only asrequired for a clear understanding of the present invention) hinged tothe slide about respective horizontal axes and movable between a closedposition and a fully-open position.

In the example shown, jaws 5, 6 of each forming assembly 4 haverespective arms 7, 8, which interact with tube 3, extend parallel to adirection B perpendicular to direction A, and are located on oppositesides of tube 3.

Each forming assembly 4 also comprises two forming tabs 9, 10 facingeach other, hinged to respective jaws 5, 6, and movable between an openposition, into which they are pushed by elastic means (not shown), and aclosed position in which they mate to define a gap defining the shapeand volume of the package 2 to be formed between them.

Each forming assembly 4 also comprises an induction sealing device 15and a cutting device 16 for respectively induction heat sealing andcutting along the mid-line of each cross section of tube 3 of packagingmaterial gripped between the relative jaws 5, 6.

With particular reference to FIG. 3, sealing device 15 comprises twopairs of induction elements 20, 21 housed in respective face seats in asupporting body 24 connected integrally by conventional fastening meansto arm 7 of jaw-5 of the relative forming assembly 4. Alternatively,supporting body 24 may be formed integrally with arm 7 of relative jaw5.

Induction elements 20, 21 are preferably connected in series (FIG. 5)and are supplied by a known current source not shown. Alternatively,induction elements 20, 21 may be connected in parallel, though thissolution is not as efficient as a series connection and fails to providefor optimum stability of sealing device 15 when induction elements 20,21 are a small distance apart.

In the example shown, induction elements 20, 21 are defined byrespective electrically conductive bars extending in direction B andarranged in pairs on opposite sides of a mid-plane R, perpendicular todirection A, of supporting body 24 of relative jaw 5.

More specifically, induction elements 20 are symmetrical with respect toplane R and interposed between induction elements 21, which are alsoarranged symmetrically on opposite sides of plane R.

Preferably, induction elements 20 have a square cross section, andinduction elements 21 a rectangular cross section.

Induction elements 20, 21 interact with tube 3 of packaging material bymeans of respective preferably rectangular active surfaces 25, 26, whichare elongated in direction B, extend in the same plane perpendicular toplane R, and have a width in direction A and a length in direction B.Active surfaces 25, which are identical, are obviously interposed, indirection A, between active surfaces 26, which are also identical and ofthe same length as active surfaces 25.

As shown in FIG. 3, respective projections 28, elongated in direction B,project from active surfaces 25 towards tube 3 of packaging material.Projections 28 may be continuous or segmented, extend alongsubstantially the whole length of respective active surfaces 25, andprovide, when heat sealing, for increasing the gripping pressure on tube3.

In a variation not shown, active surfaces 26 may also be provided withrespective continuous or segmented longitudinal projections.

An important aspect of the present invention is that the width L1 ofactive surfaces 25 is greater than or equal to the width L2 of activesurfaces 26, at least along most, preferably 80%, of the length ofactive surfaces 25, 26.

The ratio of width L1 of active surfaces 25 to width L2 of activesurfaces 26 advantageously, ranges between 1 and 4, and is preferably2.25.

The FIG. 4 graph shows the power induced (continuous line) and thetemperature (dash line) in the aluminium layer of the cross section oftube 3 gripped between a relative pair of jaws 5, 6 as a function of thedistance from mid-plane R of jaw 5 or, equally, from the mid cuttingline of the cross section.

The FIG. 4 graph refers to only one of the two halves into which thecross section of tube 3 gripped between jaws 5 and 6 is ideally dividedby plane R, it being understood that the induced power and temperaturecurves in the other half of the cross section are perfectly symmetricalwith those shown with respect to plane R.

As shown in the FIG. 4 graph, the power induced in the aluminium layerreaches a peak P1 at active surfaces 25 and a peak P2 at active surfaces26, and assumes a minimum value between each active surface 25 and theadjacent active surface 26. More specifically, peak P2 at inductionelements 21 is much higher than peak P1.

The temperature graph is a direct result of the induced powerdistribution resulting from the dimensional ratios of active surfaces25, 26 of induction elements 20, 21.

As shown clearly in FIG. 4, moving away from plane R in direction A, thetemperature in the aluminium layer of the cross section of tube 3gripped between jaws 5 and 6 falls slightly up to the outer periphery ofjaw 5 (linear portion T1), and then sharply (knee portion T2) beyond jaw5 where the packaging material does not need heat sealing. Morespecifically, along linear portion T1, the temperature falls by about15–20%, so that heat is distributed more or less uniformly along thewhole cross section of tube 3 being sealed.

In a preferred embodiment, supporting body 24 is made of aluminium-basedmaterial, and is fitted inside with two inserts 30 made ofmagnetic-flux-concentrating material—in the example shown, aferrite-containing composite material—and interposed between inductionelements 20, 21 and respective portions of supporting body 24.

Sealing device 15 also comprises two pressure pads 31 (FIG. 1) made ofheat-resistant elastomeric material, preferably nitrile rubber, andhoused in respective face cavities of matching shape formed in jaw 6 ofeach forming assembly 4 and located symmetrically on opposite sides ofmid-plane R. Pressure pads 31 of each jaw 6 cooperate with activesurfaces 25, 26 of induction elements 20, 21 of respective jaw 5 to gripand heat seal tube 3 on opposite sides of plane R.

With reference to FIGS. 1 and 2, cutting device 16 comprises asubstantially flat cutting member 32, which is housed in sliding mannerin a face seat on jaw 6 of each forming assembly 4, is movable alongplane. R, and is activated in known manner (not shown) by a hydrauliccylinder incorporated in jaw 6.

Cutting member 32 is normally maintained, by known elastic means (notshown), in a withdrawn rest position housed completely inside respectivejaw 6, and is moved by the respective hydraulic cylinder into a forwardcutting position in which it projects frontwards from respective jaw 6,engages a cavity 33 in respective jaw 5, and cuts along the mid-line ofthe relative cross section of tube 3.

Cutting member 32 comprises a platelike base portion 34 integral withthe output member of the hydraulic cylinder; and a cutter 35 thinner indirection A than base portion 34 to ensure high pressure during thecutting operation and prevent damaging the packaging material. Morespecifically, cutter 35 is connected to base portion 34 by a portion 36increasing in section towards base portion 34 and defined by concaveouter surfaces 38, and has a pointed end 37 at the opposite end.

Sealing device 15 as described above is especially suitable forpackaging units in which the cross sections of tube 3 of packagingmaterial are cut before being induction sealed.

More specifically, as shown by a comparison of FIGS. 4 and 7, the newdimensional ratios adopted for active surfaces 25, 26 of inductionelements 20, 21 provides for distributing heat more uniformly along thewhole cross section of tube 3 gripped between jaws 5 and 6, andtherefore improving sealing quality and, above all, eliminating theadverse effect of the molten heat-seal plastic material “extruding” fromthe sealing band due to local overheating.

Moreover, the change in induced power distribution along the crosssections of tube 3 for sealing, and in particular the increase in powerinduced at the outermost induction elements 21 of each jaw 5, eliminatesthe need for inserts of magnetic-flux-concentrating material in the eddycurrent bend-off regions and at the intersection between the transversesealing bands and the longitudinal seal on tube 3 of packaging material.One sealing device 15 can therefore be used for different widths of tube3 of packaging material, to produce different types of packages of thesame volume.

Clearly, changes may be made to what is described and illustrated hereinwithout, however, departing from the scope of the present invention asdefined in the accompanying claims.

1. An induction sealing device for heat sealing packaging material forproducing sealed packages of pourable food products and fed in a firstdirection; said sealing device comprising induction means interactingwith said packaging material by means of a pair of first active surfacesand a pair of second active surfaces interposed between said firstactive surfaces in said first direction: each said first and secondactive surface having a width in said first direction and a length in asecond direction perpendicular to said first direction; the length of atleast one of the second active surfaces being substantially equal to thelength of at least one of the first active surfaces, the width of saidsecond active surfaces being greater than the width of said first activesurfaces at least along most of the length of the first and secondactive surfaces.
 2. The device as claimed in claim 1, wherein the ratiobetween the width of said second active surfaces and the width of saidfirst active surfaces ranges from 1 to
 4. 3. The device as claimed inclaim 2, wherein said ratio between the width of said second activesurfaces and the width of said first active surfaces is 2.25.
 4. Thedevice as claimed in claim 1, wherein at least said second activesurfaces have respective projections projecting towards said packagingmaterial.
 5. The device as claimed in claim 1 comprising a supportingbody for housing said induction means, and which is made of analuminum-based material and has, internally, a layer ofmagnetic-flux-concentrating material interposed between said inductionmeans and the supporting body.
 6. The device as claimed in claim 1,wherein said induction means comprising two pairs of electricallyconductive portions respectively defining said first and second activesurfaces and connected in series with each other.
 7. The device asclaimed in claim 1 wherein a tube of packaging material is fed in afirst direction and filled continuously with said food product; saiddevice including at least two pairs of jaws acting cyclically andsuccessively on said tube to grip the tube at equally spaced crosssections; and wherein the induction sealing device is carried by eachpair of said jaws for heat sealing the packaging material at said crosssections.
 8. The device as claimed in claim 7, including cutting meanscarried by each pair of said jaws to cut said cross sections of saidtube in a second direction perpendicular to said first direction; saidcutting means comprising at least one cutting member having a baseportion and a cutter extending from said base portion and thinner thanthe base portion in said first direction.
 9. The device as claimed inclaim 8, wherein said cutter is connected to said base portion by aportion increasing in section towards the base portion and defined byconcave outer surfaces.
 10. An induction sealing device for heat sealinga tube of packaging material and including a cutter element for severingindividual packages as the tube advances in a first direction, theinduction sealing device comprising: opposed jaws moveable toward eachother to grip the tube of packaging material, a supporting body mountedon the jaws, a first pair of induction elements mounted on thesupporting body and spaced apart from each other to form a groove forreceiving the cutter element upon movement in a direction perpendicularto the first direction, a second pair of induction elements mounted onthe supporting body and spaced from said groove a distance greater thanthe first pair of induction elements, the first and second pairs ofinduction elements forming a planar surface for engaging the packagingmaterial to apply heat sufficient to seal the packing material, at leastone of the induction elements of the first pair being spaced apart fromboth of the induction elements of the second pair, the width of theplanar surface of the first pair of induction elements being greaterthan the width of the planar surface of the second pair of inductionelements at least along most of the length of the induction elements.11. The induction sealing device as claimed in claim 10 wherein theinductors extend parallel to each other and the ratio of the width ofthe planar surfaces of the first pair to the width of the planarsurfaces of the second pair is in the range of between 1 and
 4. 12. Thesealing device as claimed in claim 10 wherein the width of the planarsurface of the first pair of induction elements is greater than thewidth of the planar surface of the second pair of induction elementsalong at least 80% of the length of the inductors.
 13. A sealing devicefor heat sealing packaging material comprising: a first pair ofinductors extending parallel to each other and spaced apart apredetermined distance from each other, a second pair of inductors, theinductors of each pair extending parallel to each other and theinductors of the first pair being parallel to the inductors of thesecond pair, the inductors having an active surface aligned in a commonplane, the first pair of inductors being interposed between theinductors of the second pair, the active surface of each of theinductors of the first pair and the second pair extending in alengthwise direction, the active surface of each of the inductors of thesecond pair having end portions extending at least 25% of the length ofthe second pair of inductors and a middle portion located between theend portions in the lengthwise direction, at least a portion of theactive surface of each of the inductors of the first pair beingpositioned between the middle portion of the active surface of eachinductor of the second pair, and the width of the active surface of theinductors of the first pair being greater than the width of the activesurface in the common plane of the second inductors of the second pair.14. The induction sealing device as claimed in claim 13 wherein theratio of the width of said active surface of the first pair of inductorsto the width of said active surface of the second pair of inductors isin the range between 1 and
 4. 15. The induction sealing device asclaimed in claim 13 wherein the ratio of the width of said activesurface of the first pair of inductors to the width of said activesurface of the second pair of inductors is 2.25.
 16. The inductionsealing device as claimed in claim 13, wherein the width of the activesurface of the first pair of inductors is greater than the width of theactive surface of the second pair of inductors for at least 80% of thelength of the inductors.
 17. The induction sealing device as claimed inclaim 16 wherein the inductors are rectangular in cross-section.
 18. Inan induction sealing device for sealing and separating individualpackages from a filled tube of pourable product comprising means foradvancing the tube in a vertical direction, a pair of jaws moveabletoward each other to clamp the tube, severing means for severing thetube while clamped between the jaws, and a heat sealing devicecomprising: a first pair of inductors and a second pair of inductors,the inductors being mounted in one of the jaws in position to engage thepacking material of the tube, the inductors having a length extending ina direction across the width of the tube when compressed by the jaws,and the first pair of inductors being mounted in said jaw in parallelrelation to each other and spaced apart from each other, the second pairof inductors being mounted in said jaw in parallel relation to eachother and to the first pair of inductors, the first pair of inductorsbeing mounted between and spaced from the inductors of the second pairof inductors, the first pair of inductors being larger in cross-sectionthan the second pair of inductors.
 19. The induction sealing device asclaimed in claim 18 wherein the first and second pairs of inductors havean action surface, the action surfaces being aligned.